Method and apparatus for configuraton, measurement and reporting of channel state information for lte tdd with dynamic ul/dl configuration

ABSTRACT

A method of operating a time division duplex (TDD) wireless communication system is disclosed. The method includes establishing communications with a remote transceiver. A subframe configuration including static and flexible subframes is determined and transmitted to the remote transceiver. A channel state information (CSI) report is received from the remote transceiver in response to the subframe configuration.

This application is a continuation of U.S. patent application Ser. No.14/530,654, filed Oct. 31, 2014, which application claims the benefitunder 35 U.S.C. § 119(e) of Provisional Appl. No. 61/899,550, filed Nov.4, 2013, both of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

The present embodiments relate to wireless communication systems and,more particularly, to operation of a Time Division Duplex communicationsystem with dynamic reconfiguration of downlink (DL) and uplink (UL)time slots over which a user equipment (UE) communicates with one ormore base stations.

With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbolsare transmitted on multiple carriers that are spaced apart to provideorthogonality. An OFDM modulator typically takes data symbols into aserial-to-parallel converter, and the output of the serial-to-parallelconverter is frequency domain data symbols. The frequency domain tonesat either edge of the band may be set to zero and are called guardtones. These guard tones allow the OFDM signal to fit into anappropriate spectral mask. Some of the frequency domain tones are set tovalues which will be known at the receiver. Among these arecell-specific reference signals (CRS), channel state informationreference signals (CSI-RS), and demodulation reference signals (DMRS).These reference signals are useful for channel and interferencemeasurement at the receiver. Cell-specific reference signals as well aschannel state information reference signals are not precoded and aregenerated by a pseudo-random sequence generator as a function of thephysical cell ID. In Releases 8 through 10 of the Long Term Evolution(LTE) of the Universal Mobile Telecommunications System (UMTS), whichwas designed for conventional point-to-point communication, the cell IDis not explicitly signaled by the base station (called eNB) but isimplicitly derived by the UE as a function of the primarysynchronization signal (PSS) and secondary synchronization signal (SSS).To connect to a wireless network, the UE performs a downlink cell searchto synchronize to the best cell. A cell search is performed by detectingthe PSS and SSS of each available cell and comparing their respectivesignal quality, for example, in terms of reference signal received power(RSRP). After the cell search is performed, the UE establishesconnection with the best cell by deriving relevant system informationfor that cell. Similarly, for LTE Release 11 the UE performs an initialcell search to connect to the best cell. To enable multi-point CoMPoperation, the connected cell then configures the UE by higher-layersignaling with a virtual cell ID for each CSI-RS resource associatedwith each respective base station involved in the multi-point CoMPoperation. The UE generates the pseudo-random sequence for each CSI-RSresource as a function of the virtual cell ID.

Conventional cellular communication systems operate in a point-to-pointsingle-cell transmission fashion where a user terminal or equipment (UE)is uniquely connected to and served by a single cellular base station(eNB or eNodeB) at a given time. An example of such a system is Release8 of the 3GPP Long-Term Evolution. Advanced cellular systems areintended to further improve the data rate and performance by adoptingmulti-point-to-point or coordinated multi-point (CoMP) communicationwhere multiple base stations can cooperatively design the downlinktransmission to serve a UE at the same time. An example of such a systemis the 3GPP LTE-Advanced system. This greatly improves received signalstrength at the UE by transmitting the same signal to each UE fromdifferent base stations. This is particularly beneficial for cell edgeUEs that observe strong interference from neighboring base stations.

FIG. 1 shows an exemplary wireless telecommunications network 100. Theillustrative telecommunications network includes base stations 101, 102,and 103, though in operation, a telecommunications network necessarilyincludes many more base stations. Each of base stations 101, 102, and103 (eNB) is operable over corresponding coverage areas 104, 105, and106. Each base station's coverage area is further divided into cells. Inthe illustrated network, each base station's coverage area is dividedinto three cells. A handset or other user equipment (UE) 109 is shown incell A 108. Cell A 108 is within coverage area 104 of base station 101.Base station 101 transmits to and receives transmissions from UE 109. AsUE 109 moves out of Cell A 108 into Cell B 107, UE 109 may be handedover to base station 102. Because UE 109 is synchronized with basestation 101, UE 109 can employ non-synchronized random access for ahandover to base station 102. UE 109 also employs non-synchronous randomaccess to request allocation of uplink 111 time or frequency or coderesources. If UE 109 has data ready for transmission, which may be userdata, a measurements report, or a tracking area update, UE 109 cantransmit a random access signal on uplink 111. The random access signalnotifies base station 101 that UE 109 requires uplink resources totransmit the UE's data. Base station 101 responds by transmitting to UE109 via downlink 110 a message containing the parameters of theresources allocated for the UE 109 uplink transmission along withpossible timing error correction. After receiving the resourceallocation and a possible timing advance message transmitted on downlink110 by base station 101, UE 109 optionally adjusts its transmit timingand transmits the data on uplink 111 employing the allotted resourcesduring the prescribed time interval. Base station 101 configures UE 109for periodic uplink sounding reference signal (SRS) transmission. Basestation 101 estimates uplink channel quality indicator (CQI) from theSRS transmission.

Traditional wireless communication systems operate in either FrequencyDivision Duplex (FDD) or Time Division Duplex (TDD) modes. In a FDDmode, a pair of radio frequency (RF) carriers is assigned respectivelyto the downlink and uplink directions of the communication system. Incontrast, a TDD system operates by time-multiplexing uplink and downlinktransmissions within a fixed time interval on the same RF carrier. Theratio between UL and DL transmissions in the fixed time interval may beselected according to UL/DL data traffic patterns, or to supportcoexistence between dissimilar TDD wireless systems. The user equipmentin a TDD system operates in a half-duplex mode, whereby it eitherreceives from or transmits to the base station at any time instant butmay not simultaneously transmit/receive.

FIG. 2, 200 is a diagram of a Long Term Evolution (LTE) TDD system. Aradio frame of 10 milliseconds (ms) is partitioned into 1 ms subframes,where each subframe is either downlink (D), uplink (U) or a specialsubframe (S). There are seven Uplink-Downlink (UL/DL) configurationseach with different uplink, downlink and special subframe patterns. Fora cell under its control, an eNB selects one of the seven UL/DLconfigurations and broadcasts the configuration in system information.User equipment served by the eNB decodes the cell's system informationto determine the correct uplink/downlink subframe configuration for thecell.

Referring now to FIG. 3, there is a diagram of a downlink subframe inLTE. Each subframe comprises twelve OFDM symbols with Extended CyclicPrefix (CP) or fourteen OFDM symbols with Normal Cyclic Prefix (CP). Thesystem bandwidth 315 consists of a plurality of L Physical ResourceBlocks (PRB), where each PRB is composed of twelve OFDM tones calledsub-carriers. The PRB is the smallest time-frequency resource allocationunit in LTE, where data transmission to a user is scheduled on one ormultiple PRBs. Different PRBs in one subframe 301 are allocated for datatransmission to different users. Furthermore, the set of PRBs on which auser receives downlink data transmission may change from one subframe toanother.

Referring now to FIG. 4, there is a diagram of a special subframe in theLTE TDD system. The special subframe 400 consists of a downlink pilottime slot (DwPTS) 401, a guard period 402 and an uplink pilot time slot(UpPTS) 403. The guard period (GP) 402 enables the user equipment toswitch from reception mode to transmission mode. The GP duration mayalso be dimensioned to support coexistence between different TDD systemssuch as coexistence between LTE TDD and Time-Division Synchronous CodeDivision Multiple Access (TD-SCDMA). Downlink data transmission may takeplace in the DwPTS region 401 which supports between three and twelveOFDM symbols. The UpPTS region 403 consists of one or two OFDM symbolsand may be used to either transmit on the Physical Random Access Channelor to transmit SRS to the eNB.

In addition to downlink data, a base station also needs to transmitcontrol information to mobile users. This includes both common controlinformation as well as user-specific control information. Common controlinformation is transmitted to all users in the cell to maintain users'connection to the network, page users in idle mode when a call comes in,schedule random access response, and indicate critical systeminformation changes in the cell. In addition, user-specific controlinformation is transmitted to each scheduled user, for example, toindicate the frequency resources on which the UE is expected to receivedownlink data or transmit uplink data. Referring back to FIG. 3, eachLTE subframe is divided into legacy control region 306 for downlinkcontrol information transmission and data region 307 for downlink datatransmissions. The legacy control region 306 comprises OFDM symbols 1-3when the system bandwidth is greater than 10 PRBs and OFDM symbols 2-4otherwise. The exact size of the legacy control region is signaled on aPhysical Downlink Control Format Indicator Channel (PCFICH). The datachannel region 307 is located after the legacy control channel region306 and is allotted for each Physical Resource Block (PRB). The legacycontrol channel region 306 is a region to which a Physical DownlinkControl Channel (PDCCH) is mapped. The data channel region 307 is aregion to which a Physical Downlink Shared Channel (PDSCH) is mapped andcarries downlink data transmission to mobile users. Further, EnhancedPhysical Downlink Control Channels EPDCCH Set 1 309 and EPDCCH Set 2 313are frequency multiplexed with the data channel (PDSCH) 311 fortransmission to a UE. That is, EPDCCH Set 1 309 and EPDCCH Set 2 313 aremapped to the data channel region 307 together with the PDSCH 311. Thereason to locate the legacy control channel region at the beginning ofthe subframe is that a UE firstly receives a PDCCH allotted to thelegacy control channel region 306 to recognize the presence oftransmission of the PDSCH. Once the presence of transmission of thePDSCH is recognized, the UE may determine whether to perform a receivingoperation of the PDSCH. If no PDCCH is transmitted to the UE, it isunnecessary to receive the PDSCH mapped to the data channel region 307.Accordingly, the UE may save power consumed in a receiving operation ofthe PDSCH. Meanwhile, the UE may receive a PDCCH located in the controlchannel region faster than the PDSCH 311 to reduce a scheduling delay.However, because the PDCCH is transmitted over the entire systembandwidth, interference control is impossible.

The legacy control channel region 306 may not be changed to a frequencymultiplexing structure to maintain compatibility with an existing orlegacy UE. However, if the eNodeB does not allot a corresponding regionof the data channel region 307 to a UE of a previous LTE version, the UEof a previous LTE version does not receive a resource mapped to acorresponding data channel region 307. Accordingly, the eNodeB maytransmit an EPDCCH for a UE of a new LTE version in a data channelregion 307 that is not allotted to the UE. In other words, an EPDCCHbeing a control channel for a UE of a new LTE version has a structuremultiplexed with the PDSCH.

FIG. 5 is a diagram of a Physical Resource Block (PRB) pair. The eNB mayconfigure 1, 2, 4, or 8 PRB pairs for transmission to the UE. However,each PRB pair is a replica, and only one PRB pair is shown for thepurpose of explanation. Each column of the diagram of the subframecorresponds to 12 subcarriers or tones in an OFDM symbol. There are 14OFDM symbols in the subframe with a normal cyclic prefix (CP). The 3OFDM symbols on the left side of the subframe include resource elements(REs) for transmission of a legacy physical downlink control channel(PDCCH) and legacy cell-specific reference signals (CRS). These 3 OFDMsymbols are necessary for backwards compatibility with previous wirelessstandards. The 11 OFDM symbols on the right include resource elements(REs) for transmission of an enhanced physical downlink control channel(EPDCCH), and demodulation reference signals (DMRS), as well ascell-specific reference signals (CRS) and orphan or unused REs. OrphanREs may exist because the UE shall always assume that 24 REs arereserved for DMRS transmission in a PRB pair configured for EPDCCHtransmission.

While the preceding approaches provide steady improvements ininterference measurement and Channel State Information reporting forwireless communications, the present inventors recognize that stillfurther improvements are possible. Accordingly, the preferredembodiments described below are directed toward this as well asimproving upon the prior art.

BRIEF SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, there is disclosed amethod of operating a time division duplex (TDD) wireless communicationsystem. The method includes establishing communications with a remotetransceiver. A subframe configuration including static and flexiblesubframes is determined and transmitted to the remote transceiver. Achannel state information (CSI) report is received from the remotetransceiver in response to the subframe configuration.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagram of a wireless communication system of the prior art;

FIG. 2 is a table showing LTE TDD uplink/downlink configurations of theprior art;

FIG. 3 is a diagram of an LTE downlink subframe of the prior art;

FIG. 4 is a diagram of an LTE special subframe of the prior art;

FIG. 5 is a diagram of a Physical Resource Block (PRB) pair of the priorart;

FIG. 6 is a block diagram showing operation of a user equipment and abase station according to the present invention; and

FIG. 7 is a diagram showing LTE TDD fixed and flexible subframeconfigurations according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a traditional homogeneous TDD network with macro cell deployments,the UL and DL traffic patterns may be substantially static orsemi-static. Thus, a same TDD UL/DL configuration may be employed atleast for time intervals in the range of hundreds of milliseconds (ms)or seconds. However, in a heterogeneous network (het-net) with smallcell deployments, the UL and DL traffic patterns may be more dynamic innature. In addition, the proximity of the neighboring small cells mayintroduce more dynamism into inter-cell interferences, and thus mayaffect system performance and/or capacity. Therefore, wireless systemperformance may be significantly improved by a much faster adaptation ofthe TDD UL/DL configuration in response to the dynamic traffic andinterference patterns seen in a het-net. TDD Enhanced InterferenceMitigation and Traffic Adaptation (eIMTA) is an LIE Release 12 featurethat introduces a fast adaptation of the TDD UL/DL configuration bydynamically signaling a reconfiguration command on the PDCCH or EPDCCH.The rate of adaptation can be as fast as an LTE radio frame of 10 ms.However, dynamic reconfiguration of the TDD UL/DL configuration may notbe applicable to UEs of earlier LTE releases such as LTE Releases 8-11.Therefore, while a first UE may be configured to monitor downlinkcontrol channels for a change in the TDD UL/DL configuration, a secondUE of an earlier release follows the semi-statically configured TDDUL/DL configuration that is signaled in System Information Block Type 1(SIB1). Unlike conventional LTE TDD systems where neighboring cells ofthe same cellular operator use the same TDD UL/DL configuration,neighboring cells utilizing the eIMTA feature may configure differentTDD UL/DL configurations in the same radio frame. This difference mayresult in both UL-to-DL and DL-to-UL inter-cell interference.

Channel state information (CSI) is essential at the eNB for schedulingdownlink or uplink data transmission to and from user equipment.Accordingly, embodiments of the present invention describe methods forconfiguring, measuring and reporting CSI to the eNB for dynamicadaptation of TDD UL/DL configuration of a cell.

Some of the following abbreviations are used throughout the instantspecification.

CCE: Control Channel Element

CQI: Channel Quality Indicator

CRS: Cell-specific Reference Signal

CSI: Channel State Information

CSI-IM: Channel State Information Interference Measurement

CSI-RS: Channel State Information Reference Signal

DCI: DownLink Control Information

DL: DownLink

DMRS: Demodulation Reference Signal

eICIC: Enhanced Inter-cell Interference Coordination

eIMTA: Enhanced Interference Mitigation and Traffic Adaptation

eNB: E-UTRAN Node B or base station or evolved Node B

EPDCCH: Enhanced Physical Downlink Control Channel

E-UTRAN: Evolved Universal Terrestrial Radio Access Network

feICIC: Further Enhanced Inter-cell Interference Coordination

HARQ: Hybrid Automatic Repeat Request

ICIC: Inter-cell Interference Coordination

LTE: Long Term Evolution

MIMO: Multiple-Input Multiple-Output

PCFICH: Physical Control Format Indicator Channel

PDCCH: Physical Downlink Control Channel

PDSCH: Physical Downlink Shared Channel

PMI: Precoding Matrix Indicator

PRB: Physical Resource Block

PUCCH: Physical Uplink Control Channel

PUSCH: Physical Uplink Shared Channel

RE: Resource Element

RI: Rank Indicator

RRC: Radio Resource Control

SIB1: System Information Block Type 1

SNR: Signal to Noise Ratio

SRS: Sounding Reference Signal

TDD: Time Division Duplex

UE: User Equipment

UL: UpLink

ZP-CSI-RS: Zero-power Channel State Information Reference Signal

Scheduling in a wireless network is achieved by the base station (eNB inLTE) transmitting downlink control information to mobile terminals (UEin LTE). In a cellular wireless network, a base station may need toschedule transmissions to multiple mobile users at the same time. As aresult, the base station needs to transmit downlink control informationto different users simultaneously. It is also possible that the basestation may transmit different types of control information to a UEsimultaneously, such as common control information and UE-specificcontrol information.

In LTE, downlink control information bits are carried in a DownlinkControl Information (DCI) format. A DCI is channel encoded, modulated,and transmitted in a specific physical transmission channel over an airinterface. In a legacy system, DCI formats are transmitted by thePhysical Downlink Control Channel (PDCCH). A PDCCH is transmitted in thelegacy PDCCH region. Different DCI formats are used for differentscheduling purposes. DCI can be used to transmit common controlinformation to all users in a cell, UE-specific downlink controlinformation to schedule PDSCH data transmission to a UE, or UE-specificdownlink control information to schedule uplink data transmission fromthe UE to the eNB.

Table I below is a relation between DCI formats and correspondingdownlink transmission modes. The DCI formats are UE-specific, monitoredby UEs, and scrambled by C-RNTI.

DL Mode DCI format Transmission scheme Mode 1 DCI 1A Single antenna portwith cell-specific reference signal (CRS) port 0 Mode 2 DCI 1 Transmitdiversity Mode 3 DCI 2A Open-loop spatial multiplexing Mode 4 DCI 2Closed-loop spatial multiplexing Mode 5 DCI 1D Single-layer multiuserMIMO with CRS Mode 6 DCI 1B Single-layer closed-loop precoding with CRSMode 7 DCI 1 Single-layer beamforming with demodulation reference symbol(DMRS) port 5 Mode 8 DCI 2B Dual-layer spatial multiplexing with DMRSports 7-8 Mode 9 DCI 2C 8-layer spatial multiplexing with DMRS ports7-14 Mode 10 DCI 2D Coordinated Multi-Point communication, 8-layerspatial multiplexing with DMRS ports 7-14

In LTE Release 11, a new physical channel called Enhanced PhysicalDownlink Control Channel (EPDCCH) is defined to transmit downlinkcontrol information in a cell. Referring back to FIG. 3, as anadditional physical resource for control information, the EPDCCH istransmitted in a subset of physical resource blocks (PRB) in the dataregion 307 and outside of the legacy PDCCH control region 306. The eNBmay configure plural EPDCCH sets in the downlink. Each EPDCCH setcomprises a subset of PRBs which are semi-statically configured by radioresource control (RRC) higher layer signals. For each UE, the configuredEPDCCH set(s) may be orthogonal or partially overlapping. EPDCCH setsare configured in a UE-specific manner and could be identical ordifferent for different users.

Turning now to FIG. 5, there is a diagram of a Physical Resource Block(PRB) pair according to a first embodiment of the present invention. TheeNB may configure 1, 2, 4, or 8 PRB pairs for transmission to the UE.However, each PRB pair is a replica, and only one PRB pair is shown forthe purpose of explanation. Each column of the diagram of the subframecorresponds to 12 subcarriers or tones in an OFDM symbol. There are 14OFDM symbols in the subframe with a normal cyclic prefix (CP). The 3OFDM symbols on the left side of the subframe include resource elements(REs) for transmission of a legacy physical downlink control channel(PDCCH) and legacy cell-specific reference signals (CRS). These 3 OFDMsymbols are necessary for backwards compatibility with previous wirelessstandards. The 11 OFDM symbols on the right include resource elements(REs) for transmission of an enhanced physical downlink control channel(EPDCCH), and demodulation reference signals (DMRS), as well ascell-specific reference signals (CRS) and orphan or unused REs. OrphanREs may exist because the UE shall always assume that 24 REs arereserved for DMRS transmission in a PRB pair configured for EPDCCHtransmission. The subframe is also divided into enhanced resourceelement groups (eREG). The eREGs are used to form enhanced controlchannel elements (eCCEs) without regard to whether they belong to alocalized or distributed EPDCCH candidate. In the example of FIG. 3, asingle row or tone of a PRB may form one eREG so that there are 12 eREGsin each subframe per PRB configured for EPDCCH transmission.

To facilitate optimal scheduling of downlink data, a UE may beconfigured to measure and report channel state information to the eNB.The eNB configures the UE with a time-frequency CSI reference resource.The frequency part of the CSI reference resource consists of a set ofPRBs for which the CSI report is valid, whereas the time componentrefers to a subframe for which a hypothetical transmission of a datatransport block can be received by the UE with a block error ratepercentage of at most 10%. Periodic and/or aperiodic CSI reporting maybe configured for a UE, where the periodic report is transmitted on thePUCCH and the aperiodic report is transmitted on the PUSCH.

The CSI measurement is a function of the SNR that is observed by a UE.In LTE transmission modes 1-8, a UE measures the channel andinterference components of the CSI report from the transmittedCell-specific Reference Signal (CRS). In LTE transmission modes 9 and10, a UE may be configured to measure the channel part based on ChannelState Information Reference Signals (CSI-RS). In addition, a UEoperating in transmission mode 10 may be configured to measure aninterference part based on an interference measurement resource that iscontained in a zero-power CSI-RS configuration.

According to the present invention, a mix of legacy LTE (Releases 8-11)and eIMTA-capable UEs may be served by an eNB in the same cell. Thelegacy UEs determine UL/DL/Special subframe pattern according to theUL/DL subframe configuration signaled in SIB1, whereas a UE capable ofeIMTA may be configured to monitor for a PDCCH or EPDCCH conveying a DCIpacket containing a dynamic reconfiguration of the UL/DL subframeconfiguration. A valid UL/DL configuration received in a detected DCI ina PDCCH or EPDCCH must be one of the 7 LTE UL/DL configurations (FIG.2). The UE determines the UL/DL/Special subframe pattern for all radioframes within a reconfiguration time period based on the UL/DLconfiguration received in the detected DCI. Under TDD enhancedInterference Mitigation (eIMTA), the DL subframe may be static (alwaysDL) or flexible. Certain subframes have a common direction of either ULor DL across all possible UL/DL configurations. The table of FIG. 6shows that subframes 610 has a static DL direction, subframes 614 have afixed UL direction and subframes 612 are fixed Special subframes.Subframes 616 are relatively static in the context of scheduling DL datatransmission since they are either DL or Special subframes. On the otherhand, subframes 618 are flexible subframes since they are either DL orUL depending on the signaled UL/DL configuration.

Turning now to FIG. 7, there is a diagram showing communication betweenuser equipment (UE) 700 and a base station (eNB) 720 according to thepresent invention. UE 700 may be a cell phone, computer, or otherwireless network device. UE 700 includes a processor 706 coupled to amemory 704 and a transceiver 710. Processor 706 may include severalprocessors adapted to various operational tasks of the UE includingsignal processing and channel measurement and computation. The memorystores application software that the processor may execute as directedby the user as well as operating instructions for the UE. Processor 706is also coupled to input/output (I/O) circuitry 708, which may include amicrophone, speaker, display, and related software. Transceiver 710includes receiver 712 and transmitter 714, suitable for wirelesscommunication with eNB 720. Transceiver 710 typically communicates witheNB 720 over various communication channels. For example, transceiver710 sends uplink information to eNB 720 over physical uplink controlchannel PUCCH and physical uplink shared channel PUSCH. Correspondingly,transceiver 710 receives downlink information from eNB 720 over physicaldownlink control channel PDCCH and physical downlink shared channelPDSCH.

Base station 720 includes a processor 726 coupled to a memory 724, asymbol processing circuit 728, and a transceiver 730 via bus 736.Processor 726 and symbol processing circuit 728 may include severalprocessors adapted to various operational tasks including signalprocessing and channel measurement and computation. The memory storesapplication software that the processor may execute for specific usersas well as operating instructions for eNB 720. Transceiver 730 includesreceiver 732 and transmitter 734, suitable for wireless communicationwith UE 700. Transceiver 730 typically communicates with UE 700 overvarious communication channels. For example, transceiver 730 sendsdownlink information to UE 700 over physical downlink control channelPDCCH and physical downlink shared channel PDSCH. Correspondingly,transceiver 730 receives uplink information from UE 700 over physicaluplink control channel PUCCH and physical uplink shared channel PUSCH.

Once communication is established with eNB 720, transceiver 710 receivesan uplink (UL) grant in a downlink (DL) subframe. Transceiver 710 usesthe CRS or CSI-RS in one or more of the DL subframes to create a CSImeasurement report that is transmitted to eNB 720 in a subsequent ULsubframe. The static and flexible DL subframes experience differentinterference conditions. In flexible subframes, the inter-cellinterference consists of DL-to-UL and UL-to-DL interference depending onthe current UL/DL configurations of neighboring cells. Therefore, it ishighly advantageous to provide separate CSI interference reports to eNB720 corresponding to each respective subframe type in order to maximizethe DL or UL throughput. For CSI configuration and reporting by a UEconfigured for eIMTA operation, the set of DL subframes may besub-divided into two CSI measurement subframe sets denoted as CSI set 0and CSI set 1. In one embodiment of the present invention, CSI set 0 mayconsist of static subframes 610 as shown in FIG. 6, whereas CSI set 1may consist of flexible subframes 618 in FIG. 6. Other configurations ofCSI subframe sets are not precluded as the eNB may configure any subsetof subframes in a radio frame into CSI sets 0 or 1.

The present invention is directed to providing improved CSI interferencereports to eNB 720 for both static and flexible DL subframes. CSImeasurement for eIMTA operation is performed in a DL subframe asdetermined by either the UL/DL configuration signaled in SIB1 systeminformation or the UL/DL configuration dynamically signaled in a PDCCHor EPDCCH. Consequently, if a DL subframe in one radio frame isdynamically signaled to be an UL subframe in a subsequent radio frame,the UE may not perform a CSI measurement in the subframe of thesubsequent radio frame. In contrast, a legacy UE measures CSI based onCRS and/or CSI-RS only in DL subframes of the UL/DL configurationsignaled by SIB1. Therefore, to support backward compatibility, the DLsubframes of the SIB1-signaled UL/DL configuration may not bedynamically changed to UL subframes. Thus, the set of static DLsubframes include both DL subframes common to all valid LTE TDD UL/DLconfigurations as well as DL subframes according to the SIB1-signaledUL/DL configuration. An advantage of this restriction is that the timingof a HARQ-ACK feedback to a UE configured for eIMTA operation inresponse to a previous UL transmission on the PUSCH may follow the ULHARQ timing determined by the SIB1-signaled UL/DL configuration similarto previous LTE releases. Furthermore, an UL grant scheduling atransmission on the PUSCH may only be transmitted in a static DLsubframe (e.g. CSI subframe set 0). It may be impossible, therefore, totrigger a CSI report for a flexible DL subframe that is consistent withlegacy CSI timing. There are several possible solutions to this problem.First, UE 700 may provide CSI reports for both static and flexible DLsubframes whenever two CSI measurement sets are configured. This may beundesirable as it mandates a maximum feedback overhead all the time andgreatly increases complexity for UE 700. Second, the DL subframelocation may be used to determine the CSI measurement set to bereported. For example, each DL subframe in which a UL grant istransmitted is associated with one of the two CSI subframe measurementsets. Alternatively, each UL subframe in which aperiodic CSI is reportedis associated with one of the two CSI subframe measurement sets. Thesecond solution may also be undesirable, since it imposes additionalscheduler restrictions. Third, additional information may be included ina CSI request field of each UL grant to indicate which CSI subframemeasurement set is to be reported. This may also be restrictive, sinceeNB 720 can then only trigger one CSI report with one UL grant at atime.

According to a first embodiment of the present invention higher layersignaling from the Radio Resource Control (RRC) layer configures the CSIsubframe measurement set to be reported for each state of the existingCSI request field contained in a UL grant. The CSI request field in anUL grant transmitted to a UE may consist of 1 bit when the UE isconfigured for single cell operation or two bits when the UE is eitherconfigured for carrier aggregation or for CoMP operation. In anexemplary embodiment, the eNB may configure a UE by RRC signaling toreceive an UL grant with a 2-bit CSI field wherein ‘00’ indicates no CSItransmission, ‘01’ indicates an aperiodic CSI request for CSI subframeset 0, ‘10’ indicates an aperiodic CSI request for CSI subframe set 1,and ‘11’ indicates an aperiodic CSI request for both CSI subframe sets 0and 1. Other mapping arrangements of CSI reports to the CSI field in theUL grant are not precluded as the important point here is that flexibleCSI reporting configurations may be enabled by RRC signaling. Thisembodiment advantageously avoids additional UE complexity, schedulerrestrictions, and CSI reporting restrictions. A consequence of thisembodiment is that the CSI reference resource is no longer tied to thesubframe carrying a UL grant indicating an aperiodic CSI request. Inlegacy LTE systems, a DL subframe carrying a UL grant containing anaperiodic CSI request is typically the CSI reference resource andprovides for at least a 4 ms interval, including the UE processing time,before the associated UL transmission. According to this embodiment, ifone CSI report is triggered for one subframe set, the corresponding CSIreference resource in the time domain is the most recent valid DLsubframe that is prior to and no later than the subframe carrying the ULgrant and in the subframe set for which the aperiodic CSI report istriggered. If two CSI reports are triggered for two subframe sets by anaperiodic CSI request in the same UL grant, the associated CSI referenceresource for each aperiodic CSI report in the time domain is the mostrecent valid DL subframe that is prior to and no later than the subframecarrying the UL grant and in the subframe set for which the aperiodicCSI report is triggered. In other words, the CSI reference resources oftwo aperiodic CSI reports transmitted in one UL subframe may correspondto two different DL subframes and indeed, two different subframe types(static and flexible). For example, one DL subframe may belong to theset of DL subframes determined by the RBI signaled TDD UL/DLconfiguration, whereas the other DL subframe may belong to the set of DLsubframes of the dynamically signaled TDD UL/DL configuration in a PDCCHor EPDCCH.

In legacy LTE systems, a CSI process is associated with one CSI-RSresource for channel measurement and one Channel State InformationInterference Measurement (CSI-IM) resource for interference measurement.According to a second embodiment of the present invention, a CSI processfor eIMTA is associated with one CSI-RS resource and two CSI-IMresources. In one example of this embodiment each CSI-IM resource may beconfigured to measure the interference observed in static (CSI subframeset 0) and flexible DL subframes (CSI subframe set 1) respectively. Thereporting periodicity and subframe offset of each CSI-IM resource ispreferably configured by RRC signaling to match the time domain patternof each subframe set. Thus, CSI-IM 0 is used for interferencemeasurement of CSI subframe set 0, and CSI-IM 1 is used for interferencemeasurement of CSI subframe set 1. For a transmission mode where a UEmay be configured with multiple CSI processes, another embodiment of thepresent invention is to support interference measurements for bothstatic and flexible subframes per CSI process. In this case, the CSIprocessing time budget at the UE needs to be increased accordingly. TheCSI reference resource may additionally satisfy a threshold thatguarantees sufficient CSI processing time given byn_(CQI,ref)≧n_(threshold), where n_(threshold)≧4 ms. This processingthreshold may be a function of the SIB1-signaled or dynamically signaledTDD UL/DL configuration and/or the number of configured CSI processes.

In legacy LTE systems, one CSI process is associated with onenon-zero-power CSI-RS resource for channel measurement and one CSI-IMresource for the corresponding interference measurement. Furthermore,the frequency domain component of the CSI-IM resource consists ofresource elements that are a subset of the resource elements specifiedby a zero-power CSI-RS configuration. In one embodiment of the presentinvention, the UE equipment performs an interference measurement usingthe resource elements of the CSI-IM resource that are contained withinthe subframe set. To support two CSI subframe sets in eIMTA operation,different alternatives may be considered. In one alternative, theconfigured CSI-IM resource within the subframe set belonging to the CSIreference resource is used to derive the interference measurement. In adifferent embodiment of the present invention a CSI-RS process consistsof one CSI-RS resource and two CSI-IM resources associated respectivelywith CSI subframe sets 0 and 1. The periodicity and subframe offset ofeach of these CSI-IM resources can be separately configured by RRCsignaling to match the pattern of each CSI subframe set. As such, CSImeasurement for CSI subframe set 0 uses CSI-IM 0, and CSI measurementfor subframe set 1 uses CSI-IM 1. In case one CSI-IM is not fullycontained in the corresponding subframe set (e.g. CSI-IM 0 occurs insome subframes of CSI subframe set 1), CSI measurement for subframe set0 (or 1) may use CSI-IM 0 (or CSI IM 1) only in CSI subframe set 0 (orset 1).

For periodic CSI feedback on the PUCCH, it should be recognized that thePUCCH is a narrow-band channel with a small payload. In legacy LTEsystems, only one CSI report may be transmitted on the PUCCH in asubframe. It is still desirable to support periodic CSI reporting of twoCSI subframe sets when a UE is configured for eIMTA operation. The CSIreporting periodicity and subframe offset for each CSI subframe set isindependently configured by RRC signaling. For transmission modes 1-9,the CSI reference resource associated with a PUCCH transmission insubframe n is the subframe n−n_(CQI,ref) such that n_(CQI,ref) is thesmallest value greater than n_(threshold)=4 and satisfying theconditions that it is a valid DL subframe and in the CSI subframe setcorresponding to the requested periodic CSI report. For transmissionmode 10, the CSI reference resource associated with a PUCCH transmissionin subframe n is the subframe n−n_(CQI,ref) such that n_(CQI,ref) is thesmallest value greater than or equal to n_(threshold)=4 and satisfyingthe conditions that it is a valid DL subframe and in the CSI subframeset corresponding to the requested periodic CSI report. In addition, theexact value of the threshold n_(threshold) is a function of the numberof CSI processes configured for the UE.

Due to the independent configuration of periodic CSI reporting in eIMTAoperation, a collision may occur in a same UL subframe, where a UE isconfigured to report CSI measurements for both CSI subframe sets on thePUCCH. As only one report may be transmitted in a subframe, a collisionhandling mechanism is desirable. An embodiment of the present inventionconfigures via RRC signaling different priorities for different subframesets. When CSI reports of different subframe sets collide in the same ULsubframe, a CSI of subframe set of a higher priority is reported, whilea CSI subframe set of a lower priority is not transmitted. In legacy LTETDD systems, the priority rule may be based on the CSI type (rank versusCQI/PMI information), serving cell index or CSI process index when a UEis respectively configured for carrier aggregation or with multiple CSIprocesses.

Several priority rules are described in the present invention. In afirst alternative, the CSI reports are prioritized according to asemi-statically configured prioritization of the CSI subframe sets. Forexample if CSI subframe set 0 contains only static DL subframes this maybe given a higher priority compared to CSI subframe set 1 containingflexible subframes. Subsequently, the tie-breaking rule for a collisionmay be secondly, according to CSI type, thirdly, serving cell index andfinally, CSI process index.

Other arrangements of this prioritization/tie-breaking rule are notprecluded amongst the four categories, namely CSI subframe set, CSItype, serving cell index and CSI process index. For example, toprioritize spatial multiplexing transmission it may be advantageous toassign the highest priority according to the CSI type. Therefore, thefollowing prioritization rules may be configured, where in each rule thecategories are according to decreasing level of priority,

-   -   1. CSI subframe set priority—CSI type—serving cell index—CSI        process index    -   2. CSI type—CSI subframe set priority—serving cell index—CSI        process index    -   3. CSI type—serving cell index—CSI subframe set priority—CSI        process index    -   4. CSI type—CSI process index—serving cell index—CSI subframe        set priority.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling with the inventive scope as defined by the followingclaims. Other combinations will be readily apparent to one of ordinaryskill in the art having access to the instant specification.

What is claimed is:
 1. A method of operating a time division duplex(TDD) communication system, comprising the steps of: establishingcommunications with a remote transceiver; determining a subframeconfiguration including static and flexible subframes; transmitting thesubframe configuration to the remote transceiver in response to the stepof determining; and receiving a channel state information (CSI) reportfrom the remote transceiver in response to the step of transmitting. 2.The method of claim 1, comprising: signaling the static subframe byRadio Resource Control (RRC); and signaling the flexible subframe by adownlink control information (DCI) packet.
 3. The method of claim 1,comprising transmitting a plurality of bits in an uplink grant toindicate which subframe is used for the CSI report.
 4. The method ofclaim 1, comprising transmitting a plurality of bits in an uplink grantto indicate whether the CSI report is determined from the staticsubframe, the flexible subframe, or both subframes.
 5. The method ofclaim 4, comprising transmitting a CSI request in the uplink grant tothe remote transceiver in a valid downlink subframe at leastn_(threshold) subframes prior to an uplink subframe specified by theuplink grant.
 6. The method of claim 1, wherein the CSI report isdetermined in response to a channel state information reference signal(CSI-RS) resource and at least two channel state informationinterference measurement (CSI-IM) resources.
 7. The method of claim 6,wherein a first CSI-IM resource determines static subframe interference,and wherein a second CSI-IM resource determines flexible subframeinterference.
 8. The method of claim 1, comprising receiving periodicCSI reports from the remote transceiver.
 9. The method of claim 1,comprising receiving the CSI report as selected by the remotetransceiver in order of decreasing priority according to CSI subframeset priority, CSI type, a serving cell index, and a CSI process index.10. The method of claim 1, comprising receiving the CSI report asselected by the remote transceiver in order of decreasing priorityaccording to CSI type, CSI process index, a serving cell index, and aCSI subframe set priority.
 11. The method of claim 1, wherein the stepof determining is in response to Radio Resource Control (RRC) signaling.12. A method of operating a time division duplex (TDD) communicationsystem, comprising the steps of: establishing communications with aremote transceiver; receiving a subframe configuration including staticand flexible subframes; creating a channel state information (CSI)report in response to the subframe configuration; and transmitting theCSI report to the remote transceiver.
 13. The method of claim 12,wherein the static subframe is signaled by Radio Resource Control (RRC),and wherein the flexible subframe is signaled by a downlink controlinformation (DCI) packet.
 14. The method of claim 12, comprisingreceiving a plurality of bits in an uplink grant to indicate whichsubframe is used for the CSI report.
 15. The method of claim 12,comprising receiving a plurality of bits in an uplink grant to indicatewhether the CSI report is determined from the static subframe, theflexible subframe, or both subframes.
 16. The method of claim 12,wherein an uplink grant from the remote transceiver comprises a channelstate information reference signal (CSI-RS) resource and at least twochannel state information interference measurement (CSI-IM) resources.17. The method of claim 16, wherein a first CSI-IM signal determinesstatic subframe interference, and wherein a second CSI-IM signaldetermines flexible subframe interference.
 18. The method of claim 12,comprising: creating a channel state information (CSI) report for one ofthe static and flexible subframes; and selecting the CSI reportaccording to at least one of a CSI subframe set priority, a CSI type, aserving cell index, and a CSI process index.
 19. A time division duplexbase station, comprising: a processor; and a transceiver coupled to theprocessor and arranged to transmit a subframe configuration includingstatic and flexible subframes to a remote transceiver, the transceiverfurther arranged to transmit an uplink grant to the remote transceiverand to receive a channel state information (CSI) report from the remotetransceiver in response to the uplink grant.
 20. The time divisionduplex base station of claim 19, wherein the static subframe is signaledby Radio Resource Control (RRC), and wherein the flexible subframe issignaled by a downlink control information (DCI) packet.